US police say the fatal shooting of Colorado's prison boss on his doorstep may have been a targeted slaying linked to his high-profile position.

The shooting punctuated an intense debate on gun control in Colorado, coming just hours before the state's Democratic governor signed into law new firearms-control measures spurred by a rash of deadly mass shootings in the state and elsewhere.

Police said Tom Clements, 58, appointed two years ago as executive director of the Colorado Department of Corrections, was shot as he answered the doorbell at his home in a secluded wooded area near the town of Monument, 72km south of Denver.

The killing did not appear to be linked to any break-in or robbery attempt, and did not appear to be a random act of violence, said El Paso County Sheriff's Department Lieutenant Jeff Kramer.

"We are sensitive to the high-profile position in which Mr Clements served and the fact there could be people who would target him based on his position," Kramer .

Clements also spent 31 years in the Missouri Department of Corrections, where he became the No 2 official.

Kramer said that according to an emergency call for help received shortly before 9pm local time, Clements was shot after answering the doorbell. He was found dead by sheriff's deputies arriving on the scene.

His home sits on a street that is "not a major thoroughfare," Kramer said. "There's no reason to turn off onto that road unless you had business there."

Officials have begun a search for Clements' assailant, but no suspects had been pinpointed, Kramer said.

He said police were investigating the sighting of what was described as an unoccupied, "boxy" two-door sedan idling near the house about 15 minutes before the first 911 call.

The same car was reported seen a short time later travelling from the scene with a lone, unidentified occupant, he said.

Police were also looking for a woman, between ages 35 and 50, who may have been "speed-walking" on Clements' street about the time of the shooting and was considered a potential witness, Kramer said.

He said some neighbours told police they heard what might have been gunshots in the area at the time.

GOVERNOR SIGNS GUN LAWS

At a news conference, Governor John Hickenlooper, visibly shaken by news of the shooting, praised Clements as a "great friend to me" and a dedicated administrator.

At a previously scheduled event later, Hickenlooper signed legislation to extend background checks on gun buyers to private firearms sales.

He also signed legislation limiting the size of ammunition magazines that may be sold in the state.

The measures were introduced in the Democratic-controlled state Legislature earlier this year and swiftly passed, moving Colorado to the forefront of a national debate over gun violence, which was reignited by several mass shootings in 2012.

They included massacres of school children in Newtown, Connecticut, and moviegoers in suburban Denver.

A previous loophole in Colorado law that exempted firearms sales at gun shows from background checks was closed after the 1999 Columbine High School massacre in Colorado where two students fatally shot a teacher and 12 students before committing suicide.

Columbine had stood as the deadliest US public school shooting on record until 20 first-graders and six adults were slain by a gunman last December at the Sandy Hook Elementary School in Newtown.

Five months before the Sandy Hook killings, a gunman opened fire during a midnight screening of the Batman movie The Dark Knight Rises in the Denver suburb of Aurora, killing 12 people and wounding 58 others.

Hickenlooper was joined at the bill-signing by sponsors of the measures and relatives of victims of gun violence.

One of them, Sandy Phillips, whose daughter, Jessica Ghawi, was killed in Aurora, told the governor, "You've given us a real gift today, adding, "Thank you so much. You're leading the entire country."

A proposed national assault weapons ban backed by President Barack Obama ran into trouble when US Senate Majority Leader Harry Reid acknowledged there was not enough support for it.